Darry acts like a father to his two younger brothers, because their parents have died. He's a bit rough with them, saying things like, "You're both nuts," because he's still young himself, only 20 years old, but he uses parenting language when necessary: "Yeah, since it ain't a school night" (pg 13) is almost like something a parent would say, except that Darry uses slang, "ain't" instead of "isn't," showing that he's actually a young guy trying to be parental.
On page 44, Darry shows again that he really cares about his brothers:
I reckon it never occurred to you that your brothers might be worrying their heads off and afraid to call the police because something like that could get you two thrown in a boys' home so quick it'd make your head spin. And you were asleep in the lot? Ponyboy, what on earth is the matter with you? Can't you use your head? You haven't even got a coat on.
Although Darry's angry and shouting at Ponyboy, it's obvious that he cares deeply about his brother and feels responsible for him, by phrases like, "brothers...worrying their heads off," and "You haven’t even got a coat on." He cares enough about Ponyboy to notice things like whether he's dressed properly.
Darry works hard to look after his brothers, but he doesn't resent it and woudln't want them to be removed from his care ("thrown in a boys' home"). Ponyboy realizes this on page 84, when Darry says to him, "Oh, Pony, I thought we'd lost you... like we did Mom and Dad..." This was the first time Ponyboy saw Darry cry. He hadn't even cried at their parents' funeral. For the first half of the novel, Ponyboy is scared of Darry and thinks Darry hates him, but he discovers Darry really does care about him at this moment.
The course of events in the novel cause Ponyboy and Darry to become closer and closer. On page 142, Darry says, "Maybe you can be a little neater, huh, little buddy?" He has always called Sodapop 'little buddy,' but this is the first time he calls Ponyboy 'little buddy.' It shows that Darry feels closer to his youngest brother, Ponyboy, as they have a few close calls through the events of the novel.
Darry keeps his feelings for Ponyboy hidden most of the time, but near the end of the novel, Darry shows that he respects Ponyboy. He tells Ponyboy to stay in school because he is intelligent: "You're not going to drop out. Listen, with your brains and grades you could get a scholarship, and we could put you through college." (pg 148) Then again when he tells Ponyboy to catch Sodapop, knowing that even out of shape, Ponyboy is the fastest runner: "Circle around and cut him off." (pg 149) Finally, Darry shows that he's very insightful for his age, and has empathy for the other boys when he talks about Sodapop's failed relationship with Sandy: "He told me he loved her, but I guess she didn't love him like he thought she did..." (pg 148)
Darry is a pretty complex character. A young orphan raising his two little brothers, he seems at the beginning of the novel to be an angry, scary guy, but he shows by the end of the novel that he's empathetic and loves his little brothers deeply.